e2d9a8a0d8 primitives: Add an API test module (Tobin C. Harding)
8ec2d353c9 primitives: Derive Clone on witness::Iter (Tobin C. Harding)
Pull request description:
In preparation for 1.0-ing `primitives` add an `api` test module that makes an effort to verify the API surface.
This is similar to what is in `units` and what is in development for `hashes` (in #4017).
Note, there is a WIP attempt at this in #3992.
Close: #3928
ACKs for top commit:
apoelstra:
ACK e2d9a8a0d86f0e836a2a42b1803f8f9d96fde0ca; successfully ran local tests
Tree-SHA512: 5ec5c87c9aa5e86e579283a5485dcb2b3b5ae59359ae5ab96f8e6634285072bef0d0f111b6780852fd88fe29677f1a84c791a3343a0cb2b09093e77125f3962b